Human Physiology, 14th edition (2016)

Chemical Composition of the Body 25

2.1 Atoms, Ions, and Chemical Bonds

The study of physiology requires some familiarity with the

basic concepts and terminology of chemistry. A knowledge

of atomic and molecular structure, the nature of chemical

bonds, and the nature of pH and associated concepts pro-

vides the foundation for much of human physiology.

the body cells, or in the intracellular compartment; the remain-

der is contained in the extracellular compartment, a term that

refers to the blood and tissue fluids. Dissolved in this water

are many organic molecules (carbon-containing molecules

such as carbohydrates, lipids, proteins, and nucleic acids), as

well as inorganic molecules and ions (atoms with a net charge).

Before describing the structure and function of organic mol-

ecules within the body, it would be useful to consider some

basic chemical concepts, terminology, and symbols.

Atoms

Atoms are the smallest units of the chemical elements. They

are much too small to be seen individually, even with the

most powerful electron microscope. Through the efforts of

generations of scientists, however, atomic structure is now

well understood. At the center of an atom is its nucleus. The

nucleus contains two types of particles— protons, which bear

a positive charge, and neutrons, which carry no charge (are

neutral). The mass of a proton is equal to the mass of a neutron,

and the sum of the protons and neutrons in an atom is the mass

number of the atom. For example, an atom of carbon, which

contains 6 protons and 6 neutrons, has an atomic mass of 12

( table 2.1 ). Note that the mass of electrons is not considered

when calculating the atomic mass, because it is insignificantly

small compared to the mass of protons and neutrons.

The number of protons in an atom is given as its atomic

number. Carbon has 6 protons and thus has an atomic num-

ber of 6. Outside the positively charged nucleus are negatively

charged subatomic particles called electrons. Because the

number of electrons in an atom is equal to the number of pro-

tons, atoms have a net charge of zero.

Although it is often convenient to think of electrons as

orbiting the nucleus like planets orbiting the sun, this sim-

plified model of atomic structure is no longer believed to be

correct. A given electron can occupy any position in a certain

volume of space called the orbital of the electron. The orbitals

form a “shell,” or energy level, beyond which the electron usu-

ally does not pass.

There are potentially several such shells surrounding

a nucleus, with each successive shell being farther from the

nucleus. The first shell, closest to the nucleus, can contain only

Brian was diagnosed with a type of cancer called mul-

tiple myeloma, for which he received a drug called tha-

lidomide. Worried and experiencing weight loss, Brian

discovered that one optical isomer can cause birth

defects if taken by pregnant women, and that thalido-

mide rapidly converts to both enantiomers in the body.

Some of the new terms and concepts you will

encounter include:

• Stereoisomers, optical isomers, enantiomers.

Clinical Investigation

LEARNING OUTCOMES

After studying this section, you should be able to:

1. Describe the structure of an atom and ion and the
   nature of covalent, ionic, and hydrogen bonds.


2. Explain the meaning of the terms polar and
   nonpolar; hydrophilic and hydrophobic.


3. Define acid and base and explain the pH scale.


4. Identify the characteristics of organic molecules.

The structures and physiological processes of the body are
based, to a large degree, on the properties and interactions of
atoms, ions, and molecules. Water is the major constituent of
the body and accounts for 60% to 70% of the total weight of an
average adult. Of this amount, two-thirds is contained within

Atom Symbol

Atomic

Number

Atomic

Mass

Electrons

in Shell 1

Electrons

in Shell 2

Electrons

in Shell 3

Number of

Chemical Bonds

Hydrogen H 1 1 1 0 0 1

Carbon C 6 12 2 4 0 4

Nitrogen N 7 14 2 5 0 3

Oxygen O 8 16 2 6 0 2

Sulfur S 16 32 2 8 6 2

Table 2.1 | Atoms Commonly Present in Organic Molecules